There has been conventionally known, as a coating material providing excellent weatherability, a fluorine coating composition comprising a base component containing a fluorine-containing copolymer having a hydroxyl group and a curing agent comprising a polyisocyanate compound. The fluorine coating composition is used as a heavy-duty top coat or as a top coat for a cement base material.
However, a conventional fluorine coating composition containing a so-called strong solvent such as toluene and xylene had a problem that if it is directly applied to a previously coated film of a paint blended with a synthetic resin, a chlorinated rubber paint, other lacquers and the like which are changed with time, shrinkage and swelling occur, thereby not obtaining a good adhesiveness.
In particular, a two-pack fluorine coating composition, which contains a base composition comprising a fluorine-containing copolymer having a hydroxyl group and a curing agent composition comprising a polyisocyanate, is widely used because it shows excellent performance in weatherability and contamination resistance, because a hydroxyl group of the base component and an isocyanate group of the curing agent cause a crosslinking reaction to constitute a three-dimensional network.
The fluorine-containing copolymer having a hydroxyl group used for the base component of the two-pack fluorine coating composition may produce a tough film wherein the film has a high crosslinking density and is excellent in weatherability and contamination resistance, as it has a higher content of the hydroxyl group (hydroxyl value). On the other hand, as it has a higher hydroxyl group content, it has a higher polarity and becomes difficult to dissolve in a weak solvent. For this reason, a strong solvent having a high dissolving power is usually used for the two-pack fluorine coating composition. As mentioned above, when the previously coated film is repaired, the two-pack fluorine coating composition has problems such as shrinkage, swelling, adhesiveness and the like.
In order to solve these problems, for the fluorine-containing copolymer having a hydroxyl group, there has been developed a fluorine-containing copolymer having a hydroxyl group, which can be dissolved in a weak soluble wherein the weak solvent has a dissolving power lower than a strong solvent. For example, in Patent Documents 1 and 2, there has been proposed a fluorine-containing copolymer having a hydroxyl group soluble in a mineral spirit which is a weak solvent. However, since the fluorine-containing copolymer having a hydroxyl group used in Patent Document 1 uses a two-pack fluorine coating material, deterioration in solubility in a weak solvent occurs when increasing the hydroxyl value.
On the other hand, as for the polyisocyanate compound used in the curing agent, there is used an aliphatic diisocyanate such as hexamethylene diisocyanate (hereinafter referred to as “HDI”) or isophorone diisocyanate (hereinafter referred to as “IPDI”), or a prepolymer of an alicyclic diisocyanate. Since a conventional polyisocyanate compound also has a high polarity, it is required to be dissolved in a strong solvent in actual use. Therefore, in some cases, the conventional polyisocyanate had a problem that it requires countermeasures against odor and damages the primer coating film in case of repainting operations, repairing operations and recoating operations.
In order to solve the above problems, there has been developed a polyisocyanate compound which is excellent in solubility in a weak solvent. Patent Documents 3 to 7 have proposed a polyisocyanate compound obtained by urethanating or allophanating a long-chain diol or a monoalcohol, or a polyisocyanate compound obtained by allophanating and isocyanurating a long-chain diol or a monoalcohol at the same time. These polyisocyanate compounds often have an insufficient solubility in a weak solvent having a lower polarity at a low temperature.
Further, Patent Document 8 has proposed a fluorine coating composition containing a base component comprising a fluorine-containing copolymer having a hydroxyl group and a curing agent comprising a polyisocyanate compound having an allophanate group and an isocyanurate group obtained from a monoalcohol having 1 to 10 carbon atoms. However, the patent makes no reference to a weak solvent, and a fluorine-containing copolymer and a polyisocyanate compound having a hydroxyl group disclosed in the patent show no sufficient solubility in a weak solvent.    Patent Document 1: JP-A-63-199770    Patent Document 2: JP-A-2004-277716    Patent Document 3: JP-A-61-72013    Patent Document 4: JP-A-2-250872    Patent Document 5: JP-A-4-306218    Patent Document 6: JP-A-5-70444    Patent Document 7: JP-A-5-222007    Patent Document 8: JP-A-5-278240